There are various conventional diesel engines which are operated with natural gas obtained by gasifying liquefied natural gas (referred to as “LNG” below) as fuel. In recent years, slow-speed two-stroke diesel engines with high-pressure gas injection (referred to as “SSD-GI” below) are attracting attention as a measure to improve the environmental emission efficiency of existing oil-fired slow-speed diesel main engines. The SSD-GI is an engine having higher thermal efficiency and higher response than a conventional heat engine using LNG (e.g., a steam turbine), and capable of outputting power at low speed. The SSD-GI can be driven in direct connection with a propeller.
However, unlike the proven oil-fired diesel engines, a high-pressure injection technique for supplying high-pressure natural gas (about 150 to 300 bar) into a combustion chamber has not been sufficiently developed for the SSD-GI which uses natural gas as fuel. There appears to be no established technique for supplying LNG fuel.
When the SSD-GI was considered as a potential main engine for LNG vessels, a method for using boil off gas (referred to as “BOG” below) as engine fuel by compressing BOG having a substantially atmospheric pressure by a multi-stage gas compressor and cooling the BOG during or after the compressing process was studied. However, the method of compressing and cooling BOG has a disadvantage that a large facility is required and large power is consumed.
For example, PTL 1 described below (see FIG. 7 or the like) discloses a configuration in which BOG in a gas tank is compressed in two stages by low-pressure and high-pressure compressors and introduced into an engine chamber as a propulsion engine for LNG-operated vessels.
Meanwhile, recently, a BOG re-liquefaction system has been achieved in LNG vessels. Thus, BOG does not need to be used as fuel, and can be liquefied and stored. In conventional LNG vessels, there has been an effort to develop a method for using BOG as fuel from the perspective of effective use of BOG. However, the problem in employing LNG as the main fuel of a main engine has been almost solved because of the BOG re-liquefaction system. In a case in which LNG is used as fuel in vessels other than the LNG vessels, no BOG treatment is required when a pressurized LNG tank is employed.
Based on such background, LNG with excellent environmental emission efficiency is recently attracting attention as the fuel of a marine main engine or the like. Various researches and developments have been carried out to develop a method for using LNG or the like.
As a method for supplying natural gas as fuel by high-pressure injection, LNG may be heated and gasified after the pressure is boosted to a high pressure. To boost the pressure of LNG, a reciprocating pump is typically used. The reciprocating pump, which has a rotational speed of about 300 rpm, is much slower than a general electric motor, which has a rotational speed of 1800 to 3600 rpm. Thus, when the reciprocating pump is driven by the electric motor, a speed reduction mechanism is required to reduce the rotational speed to that of the reciprocating pump.
A geared or pulley speed reduction mechanism has been known as a typical speed reduction mechanism used for operating the reciprocating pump. The geared speed reduction mechanism is a speed reduction mechanism which combines a plurality of gears with different teeth numbers. The pulley speed reduction mechanism has a structure in which large and small wheels coupled via a V belt are rotated.
In a plant for re-gasification of liquefied gas, the pressure of liquefied gas removed from a storage tank is boosted to a high pressure by a pump in a liquid state as disclosed in, for example, PTL 2 described below.
In marine diesel engines, electronically-controlled engines with high environmental responsiveness which can reduce nitrogen oxides emissions or the like have been recently developed in response to a worldwide tightening of regulations on exhaust emissions from marine engines. In the electronically-controlled engines, the driving of at least one of a fuel injection system, an exhaust valve train system, a start-up system, and a cylinder lubrication system, which are driven by a camshaft in conventional engines, is electronically controlled. The electronically-controlled engines employ a method of controlling high-pressure hydraulic oil by a controller and an electromagnetic valve and thereby driving respective units of the engine.